The effect of benzodiazepines on isoniazid induced convulsions, increase of cerebellar cGMP and anterior pituitary cAMP content, was compared to the effect of muscimol and nipecotic acid (powerful GABA mimetic compounds). Benzodiazepines like muscimol and nipecotic acid delayed the onset of convulsions induced by isoniazid in doses that did not protect the animals from picrotoxin or strychnine convulsions. Moreover diazepam and muscimol have a similar order of potency in preventing the increase of cerebellar cGMP or pituitary cAMP induced by isoniazid, picrotoxin, harmaline and reserpine. In the nigro-striatal system benzodiazepines, like other GABA receptor agonists, block the activation of DA neurons induced by antipsychotics. These data suggest that the pharmacological action of benzodiazepines may be mediated by an effect on GABA neurons. The role of GABA in the action of benzodiazepines is emphasized by experiments showing that diazepam elicits positive cooperativity in the specific binding of GABA to synaptic membranes. BIBLIOGRAPHIC REFERENCES: Biggio, G., Costa, E. and Guidotti, A.: Pharmacologically induced changes in the 3'5'-cyclic guanosine monophosphate content of rat cerebral cortex: Difference between apomorphine, haloperidol and harmaline. J. Pharmacol. Exp. Ther. 200: 207-215, 1977. Guidotti. A., Naik, J. R. and Kurosawa, A.: Possible role of gamma aminobutyric acid (GABA) in the regulation of cAMP system in rat anterior pituitary. Psychoneuroendocrinology, in press, 1977.